The invention is based on a washing machine with a housing and a washing assembly suspended elastically therein, with the washing assembly containing a cylindrical plastic tub, a laundry drum mounted rotatably therein, the axis of which has an essentially horizontal alignment and a drive motor, which drives the laundry drum directly or transmits the drive from the outside via a pulley. To support the drum shaft, the end face wall of the tub has a bearing mount.
During operation of the washing machine, in particular when spinning, enormous rotational and bending forces arise, which are generated by the rotational movement of the laundry drum and transmitted onto the tub. The load of the tub increases with the size of the drum and the load quantity. The dynamics of the forces acting on the tub are dependent on the speed of the laundry drum, the force effect on the tub intensifies particularly when the drum is starting up.
In the case of the front-loadable washing machine, the drum is mounted on one side. With these washing machines known as front loaders, the bending and rotatary forces transmitted by the laundry drum have to be taken up by the bearing mount in the end face wall of the tub and transmitted and/or distributed across the whole surface thereof. The requirements and/or mechanical rigidity are particularly high with this type of washing machine. The construction of such a tub must exhibit an adequate rigidity in order to be able to withstand all loads with, for the construction of the tub, account being taken of the fact that the mechanical stress in the region of the bearing mount is at its greatest and reduces from the bearing mount across the end face wall to the tub casing.
It one assumes that the service life of a washing machine amounts to more than 10 years, a tub has to be designed such that it can fulfill all functional requirements over such a long period of time, particularly in respect of its ability to withstand the mechanical stresses transmitted by the drum and in respect of its water tightness and the corrosion resistance.
Another demand made on mass products such as washing machines is that their individual modules, such as the tub in the present instance, not only have to satisfy the functional requirements but must also be economical in terms of manufacture and problem-free in terms of recycling when the washing machine has reach the end of its service life.
To ensure that the tub is made suitably stable, plastic tubs have been developed for washing machines, in which additional stabilizing means are used in particular for the end face wall and the bearing mount. Webs and/or ribs or a metal bearing cross integrated in the plastic of the end face wall are known from the prior art as stabilizing means.
Plastic tubs with a metallic bearing cross integrated in the plastic of the end face wall are described in literature in various forms. A known method of manufacturing such plastic tubs is injection molding such that the bearing cross is firstly arranged in the mold of the injection machine and the injection itself then takes place. Insert-molding the bearing cross enables it be fitted tightly in the end face wall.
The plastic tub with a specially configured bearing cross is described in the patent specification DE 10 2005 018 190 B3. The metallic bearing cross with the sleeve-like collar for receiving the drum shaft is characterized by specially formed recesses and/or profiling, which guide the melt flow of the plastic mass during injection molding into predetermined directions so that joint lines form, which lend additional stability to the tub in a similar fashion to the ribs.
The methods for manufacturing tubs with an injection-molded bearing cross are technologically difficult and expensive. Furthermore, the injection molding method also has a series of technical problems and disadvantages, which derive from materials being used which have very different properties:
Since the said tubs are on the one hand a container made of plastic and on the other hand a bearing cross made of metal, their different material properties have a disadvantageous effect on the cooling process following injection molding such that as a result of the different expansion coefficients and the different thermal conductivity, significant stresses arise following the injection molding of the metallic bearing cross in the tub which cause the plastic to shrink when cooled to room temperature.
The forces, which act here on the bearing cross are so large that the bearing cross can deform. Furthermore, cracks can form in the plastic and result in deposits on the boundary surfaces of the two different materials. After a longer operating time, this can result in a loss of seal of the tub and loosening of the bearing cross in the end face wall.
A further problem consists in the plastic inside the material being cooled differently to the boundary surface of the bearing cross. As a result, micro holes develop between the plastic and the bearing cross, thereby impairing the connection between the materials and possibly resulting in cracks.
As an alternative to the described plastic tubs with the bearing cross integrated in the end face wall, tubs have been developed, the stabilization means of which consist of plastic. DE 20 2004 012 221 U1 describes such a tub for instance. The rear end face wall of the tub is provided with a plurality of straight, rigid ribs, which are arranged at an equal angular distance from one another and starting from the through bearing mount, which is arranged in the middle of the rear end face wall, run radially toward the outer edge of the end face wall.
The ribs are a component of the end face wall, they are formed using injection molding during their manufacture. The metallic bearing cross as an additional component is omitted. The unification of technology made possible by this method brings with it not inconsiderable potential for rationalization, it is possible to manufacture the tub with a significantly reduced technical and financial outlay.
Tubs are also known, which, in the region of the end face wall and in addition to the radial reinforcement ribs, also comprise meander, star, oval, circular or conical stress-relief profiles. The profiles are embodied such that they are well suited to forces running in a number of directions.
Tubs of this type have greater rigidity compared with the previously mentioned example. This is disadvantageous for tubs in that it results in material accumulations in the cross regions of the different stress relief profiles, which, on account of the different temperature gradients cause the material to shrink differently after injection molding. The results are material stresses within the end face wall with the resulting risk of cracks forming in the material and after longer operating time, the tub becoming untight.
Practice has now shown that the described known constructions of tubs with stabilization means made of plastic cannot be used or only used to a limited degree for larger load quantities and very high spin speeds.
Adjusting the rigidity of a tub constructed in this manner to increased requirements is restricted. The rigidity of the tub can be increased up to a certain degree, by the material intensity of the end face wall and/or the stabilization means molded therein being designed higher and stronger. This possibility of the extensive amplification of the end face wall of the tub are subject to relatively limited limits, since material bursts occur with an amplification of the end face wall and the stress relief profile, said material accumulations, as described, possibly resulting in stresses and crack formations and are thus to be prevented.
Lending the tub increased rigidity by using strong plastics is not offered for economical reasons since plastics render the tub considerably more expensive.
With modern machines with spin speeds in the drum of above 1500 revolutions and a load of more than 8 kg, with the known constructions of tubs, which are exclusively manufactured from plastic, an adequate rigidity for the strong and complicated bending and rotary loads is not achievable.
Following the trend for even higher spin speeds and higher drum capacities, the requirements in terms of stability of the tub increase, thereby further reducing the possible application for the known tub.